The invention relates to a method and apparatus for sealing vacuum insulating containers such as vacuum bottles. More specifically, the invention relates to a method and apparatus for sealing the vacuum insulating containers in a vacuum chamber.
A vacuum insulating container such as a metal vacuum bottle is conventionally sealed under vacuum conditions by one of two prior art methods depicted in prior art FIGS. 1 and 2.
As shown in prior art FIG. 1, a brazing material 3 such as a silver brazing filler metal is placed on the outer edge of an evacuation hole 2 made on the outer container 41a with suitable pitches 27. A sealing plate 40 is set over evacuation hole 2, and the container is then set into a prior art vacuum heating furnace 5. A first stage heat treatment is provided for a fixed time to remove absorbed gases. Next the temperature of the outer container 41a is raised to the melting temperature of the brazing material (800.degree.-1,050.degree. C.) to melt the brazing material so that the sealing plate 40 is brazed with the outer container 41a.
A serious problem associated with the brazing method of sealing the vacuum container is that both the outer container 41a and the inner container 41b are heated up to the melting temperature range of the brazing material (800.degree.-1,050.degree. C.). Because of this high temperature, silver plating, which is the most suitable means to prevent heat loss by radiation cannot be effectively utilized with respect to either the inner or outer containers 41b or 41a. A film of silver plating is damaged by heat at a temperature of approximately 600.degree. C., and the damage resulting from overheating results in a considerable decrease in the ability of the silver plating to prevent radiation.
In the above regard, even if an alternative low melting point solder were substituted for the brazing material 3, a problem would still occur. A stainless steel assembly comprising inner and outer containers 41b and 41a cannot be sufficiently degassed when the melting point of a solder is too low. A result of poor degassing is low welding reliability which makes this method unsuitable for vacuum sealing.
A second prior art vacuum sealing technique is illustrated in prior art FIG. 2 wherein space G is exhausted with a vacuum pump (not illustrated) through an exhaust pipe 6 installed on the bottom of the outer container 41a. Exhaustion of space G is accomplished while the container is inside prior art heating furnace 7. After completion of evacuation, the exhaust pipe 6 is pressure-welded with tools 8a and 8b for vacuum sealing.
A problem associated with the pressure welding technique is that extra insulation of the exhaust pipe 6 on the outer container 41a is required, and this makes the manufacturing process more complicated. The size of the vacuum insulating container becomes larger because of the increase of a dead space since an extra covering case (not illustrated) is required to conceal the projection portions of the exhaust pipe 6.
Accordingly, it is an object of the present invention to provide a method and apparatus for vacuum sealing a vacuum insulating container without damaging silver plating films which prevent radiation heat loss.
Another advantage of the present invention is the provision of a vacuum sealing method and apparatus which precludes the necessity of a vacuum container assembly being heated up to a temperature high enough to damage a silver plating film.
Another advantage of the present invention is the provision of a vacuum sealing apparatus not requiring extra insulation to accommodate an exhaust pipe extending from the vacuum container being sealed.